Both the rules according to which neurons achieve appropriate connections with each other and the mechanisms by which these rules are realized remain among the most active topics of neurobiological investigation. We have used two different insects, the house cricket Acheta domesticus and the fruitfly Drosophila melanogaster as material for experiments whose goals is to gain insight into these topics. The abdominal cerci of crickets are sensory structures whose receptor cells form functional connections with identified interneurons in the C.N.S. In the past we have studied how the cerci regenerate after ablation and in particular the ways in which the sensory fibers re-establish connections with interneurons. We are now examining how these connections are first established during embryogenesis. Our immediate goal is to remove the first-formed or pioneer fibers in order to assess their putative role as guides for subsequently formed axons. The wings and halteres of flies both bear many receptors whose projection patterns within the C.N.S. are different. Mutations exist which cause the interconversion of wings and halteres, and we have studied the resulting neural projections. Our goal in the coming grant period is to assess the possible role of native wing axons in the C.N.S. in guiding the axons arriving in the C.N.S. from halteres which are genetically converted into wings.